DESCRIPTION: In this proposal the Principal Investigator is proposing to investigate the mechanism(s) that may be responsible for a sustained reduction in plasma HIV-1 RNA levels despite the appearance of dual drug-resistant mutations in some HIV-1-infected individuals treated with a combined zidovudine (ZDV) plus either lamivudine (3TC) or nevirapine (NVP). The PI is hypothesizing that durable suppression of viremia may result from a combination of factors, including: (1) emergence of drug-resistant viruses that contain "suppressor" genetic mutations with HIV-1 reverse transcriptase (RT) during multi-drug therapy in vivo that restore drug susceptibility in vitro (e.g., RT mutations associated with either 3TC or NVP therapy suppress ZDV resistance); and (2) selection of drug-resistant viruses that have attenuated replicative capacity (i.e., viruses that are "less fit" than wild type). The PI is proposing to: (1) identify multiple mutations in the reverse transcriptase (RT) of clinical HIV-1 isolates that together confer dual- or triple-drug resistance; undertake detailed phenotypic and genotypic characterization of selected clinical HIV-1 isolates; confirm that dual- or triple-resistance-conferring mutations occur at the clonal level; and identify novel resistance mutations that develop during combination therapy; (2) investigate potential mechanism(s) for sustained reductions in viral burden in certain HIV-1 infected subjects over the time despite the emergence of dual-and triple-drug resistance during combination therapy. The focus will be on the evolution and the patterns of genetic drug resistance over time in selected subjects, particularly looking for the presence of "suppressor" genetic RT mutations in HIV-1 isolates that result in restored phenotypic drug susceptibility in vitro. The results of serial phenotypic and genotypic drug resistance analyses will be linked to results of other virologic assays (i.e., determination of plasma viral burden and presence of syncytium-inducing (SI) viral phenotype) to identify those virologic factors most critical for sustained response; and (3) investigate the biologic properties of clinical HIV-1 isolates derived from 3TC-treated subjects with respect to enzymatic activity, viral replicative capacity, and genetic stability. It is expected that the understanding of the mechanism(s) that contribute to sustained HIV-1 suppression may allow definition of more rationale therapeutic strategies for treatment of HIV-1 individuals.